An electron diffraction study of p-xylylene

Abstract

4514(151 For comparison. Debar et a, ' state tnat the optimirea C, symmetrical trans;tion state (w tn 2 X 2 CI) ties 40 kcal/mol aDove the unsymmetrical (161 Mclver has suggesteo tnat transition states of cycioaaoitions (and many other react'ons. as well) may genera ly oe nonsymmetrc the symmetric (C,, for example, for the Diels-Alder reaction) trans;tion state representing a stattonary po.nt wtth fwo negative force constants (J W. Mclver. Jr., J.Am Chem. SOC.. 94, 4782 (1972); Acc. Chem. Res.. 7, 73 (1974)). While this has oeen proven to De the case in MIND012 and MlhD013 ca.culations on the Diels-Alder transition state, the results reported here and those of Salem and coworkers suggest that the force constant for ant symmetric distortion (one new bond lengthen'ng wh'le the other shortens) 's slightly positive. in contraoostinction to the MIND0 results where tnis force constant is nega! ve. (171 R. S Mu liken. Because of the nLmDer of parameters involve0 in tnese techniques it IS d.ff'cuit to prove bnicn interactions are treated unrealistically. M. V. Bas-ilevsKy. Adv Chem Phys. 33, 345 (1975). nas commented on possible ,nderest mation of repJlslve forces at transit on-state distances by MlhDO 2 On tne otner hand, Goroon and borr(ers found that MIND0/3 grassy .ngerestimates ong-range Interactions of tne type wh ch resLlt n me pnenomenon of spiroconjJgation : M D Gordon, T. FLkJnaga. and H. E. Simmons. J. Am. Chem. Soc.. 98,8401 (1976). ThLs. one short-range interaction such as that present in the asynchronous transition states may well oe overestimate0 relative to tne two ,ong-range interactions present on the synchronoLs transdion states. We thank Dr. Gordon for a preprint of rhese resdts (211 Ep,ot s and Yates nave shoan that ' ar0mat.c ' molecLles have less c osezi-sne repulslon tnan 'nonaromatic ' (ana oy extension. antiaromatic molecules will have the most1 hc D. Epiotis and R L. Yates. J. Am. Chem. Soc, 98, 461 (19761. Our treatment of transition states IS stm.lararomalic ~s)nchrono,s a lowed) transition states hav ng less closed-shell repJ ston than ' nonaromat c' (nonsynchronous) (221 MlhDO, 3 gives a symmetrca transition state for the etnylene-singlet oxygen reactoon: M J S. Debar and W Thie , J Am. Chem Soc., 97,3978 (19751. 0-1 th s transition state lacks signilicant closed-shell repulsions. For OrDital-symmetry-forDidoen reactions sLch as [,2, + .2,] reaction, th s type of reason ng leaos to the expectation tnat MINDO. or other overlap-neglected. calculat ons wi I tena to give more symmetrical transition states !han over,ap-.nc lroeo calculations A R. Gregory ana M. N Paddon-Row have recently shown that CNDOlP and INDD calculations predict staoil ty for mo ecJies wnch we feel snould experience serious closed-shell repulsions: J. Am Chem. Soc.. 98, 7521 (1976) (231 We thank Professor Lionel Salem for a nelpful 0 scbssion of tnis and related points (24) Alter smmiss on of th s work Basi<evsky and coworkers reported a aeficiency n ZDO methoos in accoLnt ng for excnange effects in regions of weaK mteract...

Recent Progress in Quinoidal Singlet Biradical Molecules

“…All these biradicaloid behaviors imply the open-shell character of 1 in the ground state, which attracted the attention of theoreticians; but Coulson demonstrated, using the Hückel molecular orbital (HMO) calculation, that the ground state of 1 possesses a closed-shell quinoid form with very large free valence (that is, radical reactivity) at two terminal methylene carbons. 4 The quinoid ground state for 1 was also confirmed by 1 H NMR, 5 UVvis, 6 IR, 6,7 Raman, 7,8 photoelectron, 9 and electron diffraction 10 measurements. On the other hand, it is unlikely that a triplet biradical species causes the biradicaloid chemical behaviors, because a CASSCF (8,8)/6-31+G(d) calculation predicted a singlettriplet energy gap (¦E ST ) of 159 kJ mol…”

Methanation reaction on ruthenium thin films: a mechanistic investigation. [PhD. thesis]

“…It is difficult to comment upon these values since equilibrium constants and rate constants for surface reactions are not generally available. A value for the equilibrium constant K. has been reported on iridium thin films [105]. At 373K the value was reported to be 2.5 um 2 1 while at 473K the value was approximately 2x10 um .…”

Rocky Core Solubility in Jupiter and Giant Exoplanets

“…Measurements on Jupiter and Saturn can give us information about the internal mass distribution of these planets and hence provide information about their present-day core size, however it is poorly understood whether the initial protocore remains stable, or whether it partially or fully dissolves into the metallic hydrogen layers above and is redistributed throughout the envelope. Resolving the problem of core erosion and solubility may also be a factor in the observed enhancement of heavy elements in the outer layers of giant planet atmospheres which has been attributed to late-arriving planetesimals [2,3].…”